I recently had a major issue on one of my labs that uses a lot of virtual machines on 3rd party Hypervisors such as VirtualBox and VMWare. Every time we tried to create a new virtual machine it only offered 32-bit OS's instead of 64-bit even though my processor is 64-bit. I tried following the instructions provided on VirtualBox's forums (https://forums.virtualbox.org/viewtopic.php?f=1&t=62339), but even that didn't work. After much hacking and trial and error I found it was a combination of the Dell hardware BIOS and the new Hyper-V "security features" in Windows 10 1803+ Here are the steps I followed to finally fix this issue just in case anyone else runs into this issue when trying to host VMs on Dell hardware with a modern version of Windows 10:

Suspend Bitlocker before
entering the BIOS if you have locked your drives.

In the BIOS disable
Secure Boot

After the reboot, VirtualBox now allows
you to create new 64-bit Guests and all VMs work again.

I found that with 1803, once you turn
Core Isolation on, it’s almost impossible to turn off again. I had to
upgrade to 1809 before the slider worked again. The registry changes
listed above all definitely work, but require a reboot before the VMs are
functional again.

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Do you have hyper-v installed? If so uninstall it and go ahead and use Virtual box or VM WARE and it should work fine

What she's referring to doesn't have anything to do with Hyper-V. There are other features in Windows 10 that will take over the host's hardware virtualization, including core isolation, credential guard, device guard, and some antivirus utilities.

Yes, even with Hyper-V uninstalled, these other features will activate Hyper-V and take over as the only allowed hypervisor on your computer.

other Virtualization stacks are allowed to operate at the same time

The Windows Hypervisor Platform adds an extended user-mode API for third-party virtualization stacks and applications to create and manage partitions at the hypervisor level, configure memory mappings for the partition, and create and control execution of virtual processors.